1. Field of the Invention
The present invention relates to a heat spreader used for various devices and equipments, and more particularly to a silicon carbide based composite material having high thermal conductivity used as a heat spreader for a semiconductor device, as well as to a semiconductor device utilizing the same.
2. Description of the Background Art
There is a rapidly increasing demand in the market place for higher speed of operation and higher degree of integration of semiconductor devices recently. Accordingly, further improvement of thermal conductivity of a heat spreader for mounting semiconductor elements for the device has been required, so as to more efficiently radiate heat generated from the semiconductor elements. Further, in order to further reduce thermal strain between the semiconductor elements and other members (peripheral members) within the device arranged adjacent on the substrate, the heat spreader is required to have a coefficient of thermal expansion close to the elements and the members. More specifically, coefficients of thermal expansion of Si and GaAs which are commonly used for semiconductor elements are 4.2.times.10.sup.-6 /.degree. C. and 6.5.times.10.sup.-6 /.degree. C., respectively, and that of alumina ceramics used commonly as an enclosure member of the semiconductor device is about 6.5.times.10.sup.-6 /.degree. C., and therefore the heat spreader should desirably have a coefficient of thermal expansion close to these values.
Further, as the range of application of electronics equipment has been remarkably widened recently, semiconductor devices have come to be used in more various and wider applications. Among these, use in so-called semiconductor power device equipments including a high output AC converting equipment and frequency converting equipment has been increasing. In such devices, heat generation from a semiconductor element is several to several ten times (for example, several ten W) higher than from a semiconductor memory or a microprocessor. Therefore, a heat spreader used for such an equipment must have its thermal conductivity improved significantly, with the coefficient of thermal expansion adapted to have conformability with that of the peripheral members. Therefore, generally, the substrate has the following basic structure, for example. First, an Si semiconductor element is placed on an aluminum nitride (hereinafter referred to as AlN) ceramic substrate having high thermal conductivity as a first heat spreader. Thereafter, below the first heat spreader, a second heat spreader formed of a metal having high thermal conductivity such as copper is placed. Further, below the second substrate, a heat radiating mechanism which can be water-cooled or air-cooled is placed. By such a structure, heat is radiated without delay to the outside. Such a mechanism inevitably results in a complicated heat radiating structure. In this structure, assuming that AlN ceramics of about 170 W/m.multidot.K is used as the first heat spreader, the second heat spreader must let go the heat transmitted from the first substrate to the heat radiating mechanism therebelow. Therefore, the second substrate must have a high thermal conductivity of at least 200 W/m.multidot.K at a room temperature and low coefficient of thermal expansion of at most 10.times.10.sup.-6 /.degree. C., and more preferably, at most 8.times.10.sup.-6 /.degree. C. to attain conformability with the coefficient of thermal expansion with the first substrate.
Among the power devices, some devices generate large amount of heat when actually used, and when used with such devices, the heat spreader itself may be heated to 100.degree. C. or higher. Therefore, sometimes it is required that the substrate has high thermal conductivity at such a high temperature. More specifically, one having thermal conductivity of at least 150 W/m.multidot.K at such a high temperature is required. The larger the capacity, the larger the size of the Si semiconductor element, and the larger must be the heat spreader on which the element is mounted. For example, a substrate used for a personal computer is about the size of 20 to 40 mm square at the largest, while a substrate exceeding the size of 200 mm square has been required for a power device with large capacity. Such a large substrate must have high dimensional precision at the time of packaging, and the precision must not be degraded even at a high temperature. More specifically, when the substrate warps or deforms at a high temperature, there would be a space at the interface between the substrate and the heat radiating mechanism (a radiator, a fin or the like) positioned therebelow, decreasing the efficiency of heat radiation. In the worst case, the semiconductor element may be damaged. Therefore, that the heat spreader surely has superior thermal conductivity at a high temperature is of critical importance.
Conventionally, a Cu--W based or Cu--Mo based composite alloy has been used for such a substance. Therefore, there has been a problem that the substrate costs considerably because of the expensive raw material, and that the subtrate is heavy. In view of the foregoing, various aluminum (hereinafter simply referred to as Al) composite alloys have been attracting attention as inexpensive and light materials. Among others, Al--SiC based composite alloy mainly consisting of Al and silicon carbide (hereinafter simply referred to as SiC) is a relatively inexpensive raw material, which is light weight and has high thermal conductivity. Commercially available pure Al itself and pure SiC itself have the densities of about 2.7 g/cm.sup.3 and about 3.2 g/cm.sup.3 respectively, and thermal conductivities of about 240 W/m.multidot.K and about 200 to 300 W/m.multidot.K, respectively. When the purity and defect density are further adjusted, thermal conductivities can be expected to be higher. Therefore, these are considered especially promising materials. Pure SiC itself and pure Al itself have coefficients of thermal expansion of about 4.2.times.10.sup.-6 /.degree. C. and about 24.times.10.sup.-6 /.degree. C., respectively, and it becomes possible to control the coefficient of thermal expansion in a wide range in the resulting composite material, which provides an additional advantage.
The Al--SiC based composite alloy and manufacturing method thereof are disclosed in (1) Japanese Patent Laying-Open No. 1-501489, (2) Japanese Patent Laying-Open No. 2-243729, (3) Japanese Patent Laying-Open No. 61-222668 and (4) Japanese Patent Laying-Open No. 9-157773. Reference (1) relates to a method of melting Al in a mixture of SiC and Al, and solidifying the same by casting process. References (2) and (3) both relate to infiltration of Al in voids or pores of a SiC porous body. Of these, reference (3) is directed to a so-called pressure infiltration process in which Al is infiltrated under pressure. Reference (4) is directed to a method in which a compact or a hot pressed compact of a mixed powder of SiC and Al is placed in a mold, and subjected to liquid phase sintering in vacuum, at a temperature not lower than the melting point of Al.
The inventors of the present invention proposed in (5) Japanese Patent Application No. 9-136164 (Japanese Patent Laying-Open No. 10-335538, laid-open on Dec. 18, 1998, corresponding to U.S. patent application Ser. No. 08/874,543), an aluminum-silicon carbide based composite material having thermal conductivity of at least 180 W/m.multidot.K obtained through liquid phase sintering. The composite material is obtained by compacting a mixture of SiC powder in the shape of particles, of 10 to 70 wt %, and Al powder, for example, and sintering the compact in a non-oxidizing atmosphere containing 99 vol % of nitrogen, with oxygen concentration of at most 200 ppm and dew point of not higher than -20.degree. C., at a temperature of 600 to 750.degree. C. Further, the inventors of the present invention also proposed in (6) Japanese Patent Application No. 9-93467 (Japanese Patent Laying-Open No. 10-280082, laid open on Oct. 20, 1998), a so-called net shape aluminum-silicon carbide based composite material of which dimension after sintering is close to a practical size, having coefficient of thermal expansion of at most 18.times.10.sup.-6 /.degree. C. and thermal conductivity of at least 230 W/m.multidot.K. Further, the inventors of the present invention proposed in (7) Japanese Patent Application No. 11-28940 (corresponding to U.S. patent application Ser. No. 09/256,783), a method of manufacturing the same composite material combining atmospheric pressure sintering or pressureless sintering and HIP (Hot Isostatic Pressing). According to this method, a compact of Al--SiC based mixed powder having 10 to 70 wt % of SiC in the shape of particles, for example, is subjected to atmospheric pressure sintering in a temperature range of not lower than 600.degree. C. and not higher than the melting temperature of Al in a non-oxidizing atmosphere containing at least 99% of nitrogen gas, and thereafter the compact is sealed in a metal container and subjected to HIP at a temperature of not lower than 700.degree. C., whereby an aluminum-silicon carbide based composite material which is uniform and having thermal conductivity of at least 200 W/m.multidot.K is obtained.
Further, reference (4), that is, Japanese Patent Laying-Open No. 9-157773 discloses a method in which a mixture of Al powder and SiC powder is hot-pressed to perform compacting and sintering simultaneously. In this method, mixed powder containing 10 to 80 vol % of Al and remaining part of SiC is compacted, and the compact is hot-pressed with a pressure of at least 500 kg/cm.sup.2 at a temperature not lower than the melting point of Al. By this method, an aluminum-silicon carbide based composite material having the thermal conductivity of 150 to 280 W/m.multidot.K is obtained by this method.
Though there are only a few references regarding the copper-silicon carbide based composite material where aluminum as the main metal component is substituted for by copper, as far as the knowledge and findings of the inventors of the present invention, such a composite material can be obtained by a method almost similar to the method as described above, by substituting copper (hereinafter simply referred to as Cu) for aluminum. Pure Cu itself has the density of about 8.9 g/cm.sup.3, thermal conductivity of about 395 W/m.multidot.K and coefficient of thermal expansion of about 17.times.10.sup.-6 /.degree. C. Therefore, as compared with the aluminum based material, the resulting composite material comes to have higher density, and hence the effect of light weight is not to be much expected. By contrast, thermal conductivity of copper is higher by about 60% than that of aluminum, while the coefficient of thermal expansion is smaller by about 40% than that of aluminum. Therefore, as compared with the aluminum based material, the copper based material is advantageous in manufacturing a substrate material which requires high thermal conductivity and low coefficient of thermal expansion. Copper has a considerably higher melting point than aluminum and is heavier than aluminum, so that it is to some extent disadvantageous in view of manufacturing cost, as compared with aluminum based material.
When the composite materials as described above are to be used for a substrate of which large amount of heat radiation is required, especially for a heat spreader of which high heat radiation and larger practical size are required such as a substrate for a semiconductor power device, there still remains some problems as described in the following to be solved. Especially, when the peripheral members of the substrate have relatively small coefficient of thermal expansion, conformability with such members must as also be considered. On the other hand, ever higher thermal conductivity is required. For example, the level of thermal conductivity of a substrate used for a semiconductor power device as high as 280 W/m.multidot.K or higher will be required in the future. The silicon carbide based composite material obtained through the above described conventional methods has thermal conductivity of at most about 260 W/m.multidot.K, and the level of the thermal conductivity lowers as the amount of SiC increases. Therefore, such a material cannot be used for a substrate having low coefficient of thermal expansion.
For example, when the coefficient of thermal expansion of the Al--SiC base material described in reference (4), that is, Japanese Patent Laying-Open No. 9-157773 is to be set to at most 10.times.10.sup.-6 /.degree. C., the amount of SiC must be increased to at least 80 vol %. As a result, thermal conductivity decreases to 157 W/m.multidot.K or lower. When the same coefficient of thermal expansion is to be attained in the Al--SiC based material described in reference (5), that is, Japanese Patent Laying-Open No. 10-335538, the amount of SiC must be at least 60 vol %. As a result, thermal conductivity decreases to about 200 W/m.multidot.K. Further, when the same coefficient of thermal expansion is to be attained in the material fabricated through the method of reference (7) combining the atmospheric pressure sintering and HIP, the amount of SiC must be increased to at least 60 wt %, and as a result, the material comes to has thermal conductivity decreased to about 200 W/m.multidot.K or lower.
In the method of manufacturing an Al--SiC based composite material described in reference (1), a casting process is used in which melt Al is poured into a mold and SiC particles are dispersed and solidified. Therefore, because of the difference in density of Al and SiC, segregation of SiC particles occur in the compact when cooled, and as a result, the solidified body tends to have uneven composition. Therefore, the surface of the solidified body is unavoidably covered by a coating layer (hereafter the layer will also be referred to as Al coating layer) formed of Al or Al alloy. The thickness of the coating layer varies considerably portion by portion on the surface of the solidified body. Further, the surface portion of the solidified body consisting of the coating layer and the inner portion of the solidified body have considerably different coefficients of thermal expansion, and therefore when heat is conducted to an interface therebetween, there would be thermal stress. Therefore, when the material with the coating layer left is used as the heat spreader for mounting semiconductor elements, the substrate warps or deforms because of the generated thermal stress, resulting in cracks between the substrate and the semiconductor elements or peripheral members, deformation of the semiconductor elements or deformation or damage to the semiconductor elements or peripheral members. Therefore, the coating layer must be completely removed in advance. This removal involves processing of a portion where a phase mainly consisting of Al which is soft and ductile, and a phase containing SiC of high rigidity co-exist, as the thickness of the coating layer varies. Accordingly, the process is difficult.
In the method of manufacturing an Al--SiC based composite material described in references (2) and (3), Al is infiltrated in the voids or pores of porous SiC body. Here, it is necessary to prevent shrinkage cavity of melt Al as observed in casting steel, and to fully fill Al in the pores of SiC to provide dense composite alloy. For this purpose, generally, excessive Al is provided as an infiltrating agent on an outer periphery of the porous SiC body. After infiltration, the excessive Al eludes and is fixed on the outer periphery of the infiltrated body, and removal of this excessive Al requires considerable time and labor. In the method described in reference (5) in which mixed powder mainly consisting of Al and SiC is compacted in advance and sintered, when sintering is performed at a temperature higher than the melting point of Al, similar phenomenon is observed, though to a limited extent.
In order to prevent such elusion and fixation of Al on the outer periphery, there is one approach as described in reference (6) in which a thin layer consisting of a mixture of an elusion preventing agent and an infiltration accelerator accelerating infiltration is formed on an outer periphery of the SiC porous body before infiltration of Al. Application and removal of such layer after infiltration, however, requires time and labor.
The pressure infiltration process described in reference (3) includes the steps of placing the SiC porous body in a mold allowing uniaxial pressing, placing Al or Al alloy thereon, melting Al in vacuum and forcing Al into the SiC porous body by external uniaxial pressing. In this case, finally, the infiltrated body is gradually cooled from below with a temperature gradient. At this time, as there is large difference in coefficients of thermal expansion between the SiC skeleton and the portion filled with Al in the infiltrated body, it is likely that Al is drawn into the infiltrated body when cooled, possibly resulting in portions not infiltrated with Al (which corresponds to the shrinkage cavity mentioned above). Therefore, a complicated control mechanism capable of highly precise control of the temperature gradient for cooling and control of pressurizing heating program simultaneously is required. This means that the overall apparatus for this process inevitably becomes expensive.
The method of hot pressing in a mold described in reference (4) suffers from the following problems in production and quality. When a continuous type hot press apparatus is used, for example, it becomes necessary to provide vacuum atmosphere and to suppress flowing out of melt substance from the mold, in order to increase the temperature of the atmosphere to be higher than the melting point of Al. Therefore, when the target material of uniform composition is to be obtained while suppressing variation in compositions, a very expensive manufacturing apparatus is necessary. When a batch type apparatus is used, flow out of the melt substance from the mold can be suppressed to a higher extent as compared with the continuous type apparatus. On the other hand, however, the series of steps including loading of the compact into the mold, keeping a prescribed temperature program and cooling must be repeated intermittently, which means lower productivity.
As described in detail above, conventional manufacturing of Al--SiC based composite material involves several problems in quality and production. Therefore, though Al--SiC based composite material has been recently attracting attention as a promising material in its performance as one of the substrates of which high heat radiation characteristic is required, such as a substrate for a semiconductor power module, the composite material of satisfactory performance has not yet been obtained by any of the conventional casting process, infiltrating process, sintering process, hot pressing or combination of these processes. One possible reason may be the following. Conventionally, in order to improve wettability between Al and SiC to promote voluntary infiltration of Al melt to spaces between SiC particles, or to suppress generation of voids, it has been a common practice to add a sub composition such as Si to Al, or to use Al containing such a sub composition as an impurity. Lowering of thermal conductivity of the composite material resulting from the existence of such sub composition has been inevitable. It is noted that though SiC itself has high thermal conductivity comparable to or higher than that of Al, the conventional Al--SiC based composite material has low thermal conductivity at a region where the amount of SiC content is high.
Generally, thermal conductivity of a substance is a function of density, specific heat and thermal diffusivity of the substance, as represented by the following equation. EQU Thermal conductivity=density.times.specific heat.times.thermal diffusivity (1)
Specific heat of a composite material is determined by the composition ratio. For example, the specific heat of SiC is 0.174 cal/g.multidot..degree. C. and that of Al is 0.22 cal/g.multidot..degree. C. The specific heat of Al--SiC composite is expressed by Cp=0.174.times.V.sub.SiC +0.22.times.V.sub.Al wherein V.sub.SiC and V.sub.Al, are the volume fraction of SiC and Al respectively. Therefore, if the composition is the same, the density and the thermal diffusivity must be increased to improve thermal conductivity. The conventional Al--SiC based composite material described above has the thermal conductivity of about 200 W/m.multidot.K with the density of not lower than 99%, and therefore, in order to improve thermal conductivity, it is particularly necessary to improve thermal diffusivity.
It is considered that the thermal diffusivity of the Al--SiC based composite material is determined by the thermal diffusivities of Al and SiC as well as the state of adhesion at the interface between the two phases. The degree of adhesion at the interface between the two phases basically improves as the density becomes higher. Therefore, the most important factor to increase thermal diffusivity of the Al--SiC based composite material is to increase thermal diffusivity of the two composition phases, especially that of SiC phase.